A novel mutation in BBS7 gene causes Bardet-Biedl syndrome in a Chinese family.

Purpose To describe the clinical features of and identify a novel mutation in Bardet–Biedl syndrome 7 gene (BBS7) in a Chinese family. Methods Nineteen individuals at risk for inheriting Bardet-Biedl syndrome (BBS) in a Chinese family participated in the study. Physical examination was performed and blood was drawn for DNA extraction. Linkage analysis was conducted for all known BBS loci, and mutation screening of BBS7 gene and BBS12 gene was performed. Results A Chinese family with inherited BBS was identified. After performing linkage analysis on all 13 known loci, we found the disease phenotype of a Chinese family with BBS linked to a locus where BBS7 and BBS12 genes locate. Conclusions This study describes a novel mutation in BBS7 causing BBS in a Chinese family. This is the first report that a mutation in a BBS gene causes BBS in a Chinese population. These results expand the spectrum of human disease associated with mutations of BBS7 since the initial three mutations in BBS7 were first identified in 2003.


RESULTS
Clinical findings: A Chinese consanguineous BBS family was identified in southwest China. Of the 19 members at risk for inheriting BBS in this family, five were diagnosed with BBS ( Figure 1). Two affected patients died of severe fever when they were five years of age, and one affected patient died from an accident. The two living affected patients displayed gross obesity in relation to the average height of adult Chinese men. In addition, these patients exhibited the following features: atypical retinal dystrophy with attenuated vessels, choroidal sclerosis-type change, severe optic disc pallor and severe macular dystrophy (Figure 2), obviously constricted visual fields, severe abnormalities of color vision, reduced rod and cone electroretinograph (ERG) response, refractive errors of −10 (IV:2) and −8.0 (IV:4) diopters, polydactyly of hands and feet, hypogenitalism (small testes and genitalia), learning difficulties, and renal abnormalities. The detailed clinical information for these two affected patients is listed in Table  1. Based on their symptoms, the affected patients were given a diagnosis of BBS [1,2].
Genotyping and sequencing analysis: The genotyping results showed that the BBS7/BBS12 encompassed STR markers, D4S402, D4S427, and GATA30B11, completely cosegregated with the phenotype in the family (Figure 1). But no linkage was found to any other known BBS loci. No mutation was found in BBS12. Mutation screening in the 19 exons of BBS7 found a 1666 A>G mutation in exon 15, which resulted in an amino acid substitution from serine to arginine at position 556 ( Figure 3). This mutation was absent in 100 normal matched controls.

DISCUSSION
It is clear that BBS is a pleiotropic, autosomal recessive disorder. The cardinal phenotype in this family was identical to most cases described previously [1,2] except that the ocular phenotype of this family showed macular dystrophy instead of a typical retinitis pigmentosa in BBS, further indicating the variant retinal dystrophy in different BBS families. The ocular manifestations of BBS included an early and usually severe rod-cone dystrophy that is detected by ERG and causes central and peripheral visual loss by the second to third decades of life. More than 70% of the affected individuals are legally blind by this time. High myopia associated with choroidal sclerosis-type changes in our family further confirmed the association of a myopia refractive error trend with BBS [19,25].
To date, 13 loci and 14 genes have been mapped and identified since the first BBS gene, MKKS, was identified in 2000 [2,18,25]. CCDC28B modifies the expression of BBS phenotypes in the patients who have mutations in other genes [2]. Mutations in BBS1 constitute the most mutations in the known BBS families (about 23%). Mutations in BBS10,BBS2, and BBS12 account for 10%, 8%, and 5% of all mutations, respectively. Other known BBS genes are rarely mutated in the known BBS families [10,19]. The mutations in these 14 genes probably affect both transport and paracrine signals, such as the planar cell polarity pathway, and cause the pleiotropic defects in affected BBS patients [2,10,12,16,18,[20][21][22]26].
In 2003, Badano, et al. [17] performed phylogenetic and genomic studies in which they used the human and zebrafish BBS2 peptide sequences to search dbEST and the translation of the draft human genome. They identified a novel gene, BBS2L1. BBS2L1 mutations cause BBS, defining a novel gene for this syndrome, BBS7. BBS1,BBS2, and BBS7 share a distinct substructure, and they may belong to a distinct subfamily of proteins [2,17]. Two transcript variants encoding distinct isoforms have been identified for this gene. The length of isoform 1 is 43 amino acids longer than that of isoform 2. Isoform 2 is ubiquitously expressed, whereas isoform 1 is expressed in retina, lung, liver, testis, ovary, prostate, small intestine, liver, brain, heart, and pancreas. BBS7 may play a role in eye, limb, cardiac, and reproductive system development. Blacque et al. [21] showed that mutations in the Caenorhabditis elegans bbs-7 and bbs-8 genes cause structural and functional defects in cilia. They further demonstrated that BBS7 and BBS8 are required for the normal localization and motility of the intraflagellar transport (IFT) proteins so the BBS7 protein plays an important role in the assembly as well as function of IFT particle components. These findings indicate that the cardinal and secondary symptoms of BBS may result from cilia [21]. How the mutation in our family caused BBS is not understood as this mutation is located closer to the C-terminal of BBS7 compared with the three mutations reported previously. The initially  identified three mutations are located in a predicted βpropeller region of BBS7 that is conserved between BBS2 and BBS7. These findings may indicate that the β-propeller region of BBS7 and the region involving the mutation identified in this study play a critical role in the IFT system. The mutations in these regions probably disturb the localization and motility of the IFT proteins [17,21]. BBS is a rare development disorder, and 14 genes that cause BBS have been identified. However, because of the significant clinical and genetic heterogeneity of BBS, there are still unknown genes involving at least 25%-50% of BBS families [10,15,16,27]. Studies done in more families, especially from different ethnic populations, will be useful to help identify other BBS disease-causing genes and mutations and to better understand the disease mechanism of BBS. Because no BBS mutation has been identified in a large Chinese population, the novel BBS7 mutation reported here will expand the insight into our understanding of BBS.